Method and apparatus for early strength testing of in-place concrete

ABSTRACT

Reduced scatter in strength determinations of hardening inhomogenous materials such as concrete is achieved by in-place screen-rejection of coarse-particle ingredients from test bodies evaluated by variations of the tensile-shear or pull-out method. Non- or semi-destructive tests on specimens or structures at any age are described in connection with construction safety, structural load-bearing capacity, evaluation of materials and systems. Use of flexible filler in the pull-out ring extends the applicability of the test system into the interior of mass structures. Portable apparatus requires no utility services.

United States Patent [1 1 Kaindl Jan. 21, 1975 METHOD AND APPARATUS FOR EARLY STRENGTH TESTING OF lN-PLACE CONCRETE Inventor: Franz Kaindl, Innsbruck, Austria [73] Assignee: Owen Richards, Chevy Chase, Md.

a part interest [22] Filed: June 8, 1973 [21] Appl. No.: 368,325

[30] June 12. 1972 Austria .5044/72 [52] US. Cl 73/88 C [51] Int. Cl. G01n 33/38 [58] Field of Search; 73/88 C, 88 R; 264/228 [56] References Cited UNITED STATES PATENTS 2,356,327 8/1944 Lebus 220/88 A X 3,541,845 11/1970 Hansen 73/88 C UX 3,595,072 7/1971 Richards 73/88 C UX Primary ExaminerJerry W. Myracle Attorney, Agent, or FirmLaurence R. Brown [57] ABSTRACT Reduced scatter in strength determinations of hardening inhomogenous materials such as concrete is achieved by in-place screen-rejection of coarseparticle ingredients from test bodies evaluated by variations of the tensile-shear or pull-out method. Nonor semi-destructive tests on specimens or structures at any age are described in connection with construction safety, structural load-bearing capacity, evaluation of materials and systems. Use of flexible filler in the pullout ring extends the applicability of the test system into the interior of mass structures. Portable apparatus requires no utility services.

29 Claims, 16 Drawing Figures sum 2 or 6 PATENTH] JANE] I975 FIGJB Priority for this invention is claimed due to Austrian patent application No. 4A 5044/72, filed June 12, i972 by Franz Kaindl.

This invention has two goals:

1. Early and precise strength test of materials and structures, particularly concrete of any aggregate size.

and

2. Measurement of field strength of materials and construction procedures in the field at reasonable cost, with hand-portable equipment and without requiring utility resources, artificial-atmosphere curing facilities, massive stationary test machines and delays that endanger safety and economy, especially by use of the pull-out (tensile-shear) test.

Strength determined by this method is a measure of in-place strength of the material tested, such as concrete. The strength number reported is directly related to structural strength, the strength of the building element in use, and so to rational definition of safety and cost risks.

Prior art strength testing of concrete is dominated by the 6 X 12 inch cylinder specimen, tested in the laboratory after curing for 28 days at ideal conditions of 90+% relative humidity and 73.4:3F. The strength number reported is a potential strength used to establish acceptability of ingredient materials such as cement, aggregate and chemical admixtures; it is not a measure of in-place safety or cost.

Prior art variations of the potential strength cylinder test include subjecting that standard cylinder to curing similar to that of field structures and to tensile strength tests of artificially cured specimens. Other variations include curingthe cylinder in the field under various elaborately described conditions, which are generally recognized as resulting in strength numbers variably higher than in-place actuality.

This inventions emphasis on pull-out or push-in tests involves measuring force to rupture within the concrete about a conic frustum of surface defined by a forcing ring and a coaxial piston ram.

Concrete tested by this invention may be in portable specimens tested. under the same curing conditions as the potential strength specimens.- If this is done, results of the two testswill correlate very closely. An example is the work done at the'Bureau of Reclamation of the U.S. Dept. of the Interior in Denver, Colorado in 1972-73: coefficients of correlation were 0.97* for cast slabs and 0.87 for shotcrete panels. Another example is the Feb., 1972' program of the National Sand and Gravel Association National Ready Mixed Concrete Association at their laboratory at the University of Maryland; results of tests of three mixes are plotted:

Tradition ahpfactii has bn'f'td design for 45 percent of potential compressivestrength or less, depending on additional safety factors. Traditional practice thus assumes that if av given concrete specimen in the laboratory gains strength a a given rate, field in-place strength (whatever its number) will gain at the same rate. The wastefullness of the45 percent designfactor is widely recognized. The questionable assumption of the equal rates of strength gain awaits wider availability of data on in-place strengths.

* LO would represent perfect correlation.

Age Cement in of Test lbsJcu. yd. Days 3000 r o 376 as; 7 X 470 a & 7

5 0m 2 0 B m a, S 52 H =2 l 5mm as I l l l PULL-OUT STRENGTH PSI Accelerated test methods have been developed to the determination in the laboratory of potential strength. These and other test methods are standardized by the American Society for Testing and Materials. Use of and effective codification of these standards await action by the American Concrete Institute.

The Absurd Present editorial in the May, 1973, ACI Journal, points out the absurdity of building a 15- story concrete building frame in eight days and having to wait twenty-eight days for determination of concrete strength. The absurdity is doubly apparent when it is recognized that the favored ASTM C684 24-hour test, now unapproved by ACI despite its availability as a standard for two years, still applies to potential strength and not to in-place strengths.

An important concern of all test methods is variability or scatter of results. In the interest of reducing such variability, it has long been the custon of ASTM standards to require moist or in-water curing of concrete strength specimens. Sometimes this artificial curing increases reported strengths; sometimes reported strength is decreased. Effect on scatter is also controversial. Many persons, including these inventors, increasingly feel the issue of artificial curing is not significant; it is significant to test in-place concrete as cured in place. I

Aggregate size also has an important effect on scatter of reported strength results, as long and widely recognized in the industry. The inventors, in previous patents and statements, have reported this also applies specifically to pull-out tests, as have B. G. Skramtajew (AC1 Journal Title 34-15, Jan-Feb. I938); P. Kierkegaard- Hansen (U.S. Pat. No. 3,541,845) and his Technical University of Denmark colleague, H. Krenchel; T. P. Tassios, National Technical University of Athens Report 21, 1968; and others. Similar effects apply to other strength and quality tests, such as drilled cores, flexural and tensile specimens, impact (Swiss Hammer) tests, indentation (Windsor Probe, l-lilti) and other tests.

Long-established procedure for reducing scatter attributable to coarse aggregate is to remove the coarsest particles by hand picking during molding of specimens (ASTM C 192-69, 3.4, Making and Curing Test Specimens in the Laboratory) and by screening (ASTM C 172-71, 4.1, Sampling Fresh Concrete). Note of the latter method observes that the reported strength, based on test of a cylinder of the undersize concrete, is usually greater than that of the total concrete, based on a cylinder with a diameter at least three times that of the coarsest aggregate particle.

That note also observes that screening reduces the total air content and increases the net air content in the screened undersize fraction. If true, and provable, the higher air-content of the undersize fraction should reduce its reported strength, thus tending the opposite way to the higher reported strength expectable (according to some) from test of a smaller diameter cylinder.

These factors may lead into the classic contoversy regarding D. A. Abrams water-cement ratio law and the revival of criticism of its application as represented by S. Walker and D. L. Bloem (ACI Journal, Sept., 1960, Effects of Aggregate Size on Properties of Concrete), H. J. Gilkey (AC1 Journal, April, 1961, Water-Cement Ratio Versus Strength Another Look) and many others.

Whatever the truth (if such exists) of this w/c ratioaggregate/specimen size complex may be, use of a lowcost, simple and rapid test by the pull-out method offers the best hope of solution. When it is further considered that the pull-out may be used in the field and in non-destructive testing of in-place structural concrete, the importance of this and related inventions begins to become clear.

A truly non-destructive tensile-shear test demonstrates in-place strength in excess of a required minimum when specified force is maintained for a given time, say 20 seconds, without failure. Even in cases when incipient rupture has taken place, immediate release of load has made it possible to retest at later ages and (due to autogenous healing) demonstrate strengths comparable companion units not theretofore tested.

The strength relationship between cylinders of 1 /2 in. screened concrete and diamond-drilled cores of the original concrete, containing 3-in. and 6-in. aggregate, was essential identity, according to B. Mather and W. O. Tynes of the US. Corps of Engineers Waterways Experiment Station (AC1 Journal, Jan., 1961, Investigation of Compressive Strength of Molded Cylinders and Drilled Cores of Concrete"). The smaller the core diameter, the less the precision. So, for equal precision or confidence in the reported strength, one has to go to the expense of testing many small cores or relatively fewer large cores.

apex angles of perhaps 90-120 (360 defining a circle).

The Japanese (T. Yoshida, University of Tokyo, 1942, and Japanese Society for the Promotion of Science, 1948) and B. Tremper (Washingtion State Highways, ASTM Proceedings, 1944) patterned test programs after the 1938 Russian report. They also use relatively large rings and got variable apex angles in the 100140 range. Tremper reported a coefficient of correlation between pull-outs and cylinders of 0.97 and respective coefficients of variation of 9.6 and 8.4 percent for 4,6,9 and 14 days.

The inventors experience has persuaded him that relatively smaller rings, that by contact force apex angles of 50-90, have many advantages. Work by F. C. McCormick (University of Virginia, 1970), H. Krenchel (1970), the Bureau of Reclamation (1973) and others appear to confirm this.

The same and other work in the US. and in Europe indicate minimum scatter with pull-out strength results when the apex angle is between 63 and 73. A typical configuration might be of the following dimensions: loading piston head diameter of 30 millimetres (mm) (approx. 1.18 in.); counterforce ring inside diameter of mm (2.76 in.); and conic frustum height spacing between those two concentric and parallel members of 30 mm. This arrangement would have an apex angle of about 67.4, based on tan a/2 (7-3)/2 X 3 2/3.

Strength scatter depends upon individual conditions, naturally. European tests with 17 mm coarse aggregate have had 2 percent scatter, using the above configuration. Based on similar experience, scatter might increase by about 5 percent, if at all, when maximum aggregate of 20 mm is increased to 30 mm.

Canadian Mines Branch tests in Ottawa (V. M. Malhotra, Dept. of Energy, Mines and Resources Report 1R 72-56, Nov., 1972; Evaluation of the Pull-Out Test to Determine Strength of ln-Situ Concrete) tested 46 pull-outs on five different strength mixes. Crushed limestone coarse aggregate had 3 percent retained on three-fourth in. (20 mm) sieve. Ages of test were 3, 28 and 91 days. Piston was 57 mm (2.25 in.), ring 127 mm (5.0 in.), height 53 mm (2.08 in.), giving apex angle of 67. Pull-out coefficient of variation was 3.0 percent average, compared to 2.6 percent for lab-cured 6 X 12 in. compression cylinders. Loads went up to 29,000 lbs. force.

If one accepts the 1972 Canadian pull-out results as a valid measure of in-place strength of concrete, then field cores averaged 1 percent higher and the laboratory cylinders, 6 percent higher. However, the cores varied about 30 percent from the in-place average, and cylinders, 22 percent; the range was from 45 to 147 percent for cores and55 to 134 for cylinders. For relatively strong mixes (500 to 630 pounds of cement per cu.yd. and 4,500 and 6,000 psi at 28 days, based on the lab cylinder), the cores ranged from 88 to 147 percent (average 110 percent) and the cylinders, 106 to 134 percent (124 percent). By contrast, for the leanest of five mixes (320 lbs/cu.yd. cement, 28-day lab strength of 2010, core of 2150 and pull-out psi X 4 of 2060) gave lab strength of 5598 percent (av. percent) and cores of 45-104 percent (av. 80 percent). This mix had pull-out strengths X 4 of 2,100, 2,060 and 2,360 psi at 3, 28 and 91 days (V 2.5, 3.4 8L 5.0%).

A similar configuration, tested at a state highway site, gave results tabulated below, showing coefficients of variation:

5 6 Age Pull-Out 6 X 12 in. Cylinder Strengths, PSl (days) PSl X 4 V Field Cured V,% Labcured V,%

l I188 239 8.4 l42l 7.9 3 2380 1.8 1942 5.5 3092 11.1 7 3220 6.8 3007 3.2 4609 3.6 14 4200 L0 4500 2.7 5250 3.0

While pull-out results may be of interest in estimating probable compressive strengths, the fact that European tests have shown that the ratio of pull-out: flexural strengths is roughly 1:1 may be of at least equal importance. The value of in-place tensile-shear (pull-out) strength determinations may arise from the fact reported by C. E. Kesler (ASTM STP 169-A, 1966, Significance of Tests and Properties of Concrete and Concrete-Making Materials, p 169) that compressive strength increases as the specimen dries, whereas a modulus of rupture (flexure) specimen decreases in strength.

A more detailed analysis of this is given by P. D. Cady et al. (Tensile Strength Reduction of Mortar and Concrete Due to Moisture Gradients, ACI Journal, Nov., 1972), on the basis of mortar briquet and splitting tensile tests. As much as 35 percent of the tensile strengths of standard moist-cure specimens was lost at intermediate degrees of saturation (30-70% R. H.) and rapid drying rates such as might apply in-place.

Similar findings were reported in Full-Scale Testing of New York Worlds Fair Structures, Vol. II, sponsored by the Building Research Advisory Board National Academy of Science. Core compressive strengths ran about 50 percent over design about 90 percent of the time, yet in actual testing to destruction, llexural capacity was not reached and ultimate strength was governed by shear. Loads were lower than predicted.

The importance of testing for load-bearing capacity in place has been demonstrated with highly accelerated shotcrete within several hours after placement in the new Washingtion, DC. Metro subway tunnels, in Austrian hydoelectric and Alpine highway tunnels. This inventions method and apparatus apply with equal validity to gunite; mortar; coarse-, intermediateand finegrained aggregate concrete; so-called pumpcrete, gascrete, cellular, foam and lightweight aggregate concretes; and similar synthetic resin-like materials which harden from an initially plastic mass.

Strength determination as soon as possible after hardening of such masses is rapdily becoming a necessity. Because of the relatively delayed applicability of prior art in-place tests like impact hammers (1,800 psi or 130 kg/cm and later), rapid sequence construction steps, such as safe load-bearing capacity tests for timing of form and shore removal, must be undertaken later than true in-place strength growth would permit. Due to high first-and rental costs of forms, particularly travelling forms and shoring supports, reduction in use time results in substantial cost cuts.

Such savings were restricted to shotcrete and shallow areas of concrete construction in this inventors Austrian Pat. No. 240,077 of 1965 and Tonindustrie- Zeitung article, June, 1966, entitled Towards Quality Testing of Early Strength of Shotcrete." Similar surface-area restrictions applied in the 1970 US. Pat. No. 3,541,845, issued to P. Kierkegaard-Hansen. US Pat. No. 3,595,072, 1971, issued to O. Richards, got into the more deeply buried pull-out piston, which might be tensioned from either surface of a wall or slab. Complications of such greater depths of embedment, such as eliminating variations due to coarse aggregate in the immediate test location, have been developed in the present invention.

Adaptation of traditional techniques of coarse aggregate removal from the immediate test location by incorporating mechanical coarse-particle removal apparatus in place is a primary feature of this invention.

A variation of this invention is to mount such an inplace screen apparatus, perforated plate, grizzly-bar or similar obstruction in the line of delivery or forming of plastic material, such as concrete, in a construction body. The screen-passing fraction may then be molded and tested at that location or elsewhere, as desired. The advantage of this variation is the requirement of no special equipment. Locations involved include chute and form openings or other accessable points.

Another form of the invention is locating screens in the intended area of test, so only undersize particles can penetrate into the immediate volume to be tested. The advantage of this method is the lack of disruption to normal placing routine; the volume to be tested is filled automatically.

A further variation is detachable fastening of screens to forms, giving the advantage of simple disposition of oversize. Tests are possible through formwork openings or after form removal. As the screen itself is embedded in concrete, screen-to-form fastenings must be disconnected before testing.

Another variation is combining screens and tests hardware on form openings before concrete placement. Here the advantage of the combination reusable plate optional screen/test units package is that test and test time decisions can be made even after completion of formwork erection. Where all formwork members are so equipped, testing points are discretionary, as a pull-out ring or test apparatus may be brought to any point.

Another variation is mounting of specific quality test hardware, such as piston insert and counterforce ring, fixed on plate in frustum-defining orientation. The piston is movable towards the ring to cause rupture. This offers the advantage that immediately upon mass set or hardening, one already knows the frustum area; the force and strength and determinable at once.

Another version of the invention is to emplace, or allow for future emplacement, of quality test hardware, such as piston and ring, on a combination reusable plate with or without a screen. This step has the advantage that if aggregate rejection apparatus is not required at a particular stage, or if it is desired to investigate correlation/aggregate size/number of tests parameters, one may save the cost of a screen apparatus.

Another variation is to use such a plate, suited for filling of a form opening or for substitution of a section of a form, for the mounting or fastening in-place of test hardware, such as ring or piston, by means of platehardware detachable fastenings, thus achieving time savings.

A further form of the invention is to attach, by a detachable device, to such a plate a pull-out or push-in shaft guide tube, to which in turn such hardware may be detachably held in place during concrete placement, in addition to such hardware being optionally detachably fastened to reinforcing steel or other rigid objects in the concrete placement space, thus facilitating optional non-use of screen.

According to a further version of the invention, the ring is positioned nearer the future concrete surface than the piston and the piston is moved towards the ring. The advantage in this is that only the piston need be installed beforehand; the ring need be positioned only at time of test. Further, exerting a pull-force to depth is better than a thrust force, as the latter would require an especially designed thrust shaft to resist buckling.

Another version of the invention is to position the piston closer to the future concrete surface than the ring and to force the piston towards the ring. This is advantageous as this configuration requires no piston shaft, since the force-originating apparatus may act di rectly against the piston. It also makes unnecessary a buckling-resistant shaft.

Another version is to fill the ring opening with an easily collapsable dummy which will yield to intrusive movement of the ruptured test frustum. The advantage of this is that it makes possible tensile-shear tests in the interior of concrete.

Another version of the invention is that the relative motion of the piston and shaft towards each other may be generally parallel rather than perpendicular to the concrete surface. This versions advantage is that both piston and ring can be simply installed, secured or removed and replaced from outside the mass under test; dummy replicas of hardware may be inserted and replaced by working apparatus if and when tests are desired.

A further form of this invention provides for the direction of movement of the piston towards the ring, when generally parallel to the future concrete surface, to be inclined with that direction towards the inside of the test mass. The advantage of this step is to inhibit outward premature spalling of the test mass and variable results.

A further form of the invention is for the test force, acting generally parallel to the mass surface, to be transmitted by a device acting roughly parallel to the mass surface; this device is pressed against the test mass by a guided roller arrangement. Here the advantage is inhibiting premature outward spalling.

A further form of the invention is to attach, either permanently or detachably, test hardware such as piston and ring to forming or to a portable form section which may be detachably fastened in form openings. The advantage of this step is simple mounting of test apparatus. If hardware is to be installed at some depth inside the mass under test, that hardware must be held firmly in the desired position during attrition of construction and concrete placement. Such hardware may include one or more of the following: screens, piston shafts, rings, piston shaft guide tubes, form-closing fasteners, access or inspection doors and other devices of use in execution of the method. Access doors may be used for the purpose of obtaining sample quantities of material or, after concrete hardening, conducting impact-rebound, indentation or other tests on the concrete surface.

Another version of the invention is to provide dummy displacement elements on the inside of forms, reusable plates or portable form sections for the purpose of creating recesses into which can be inserted test hardware. Such hardware may include ring. piston. and hydraulic load cell for exertion of test force generally parallel to the concrete surface. The advantage of this step is the inside application of test force, the direction of which can be accurately controlled. and avoidance of outward spalling.

Apparatus covered by this invention includes a screen grid, the outer rods of which are parallel or at an acute angle to the placing direction of the concrete. thus facilitating deflection and rejecting coarse aggregate particles and inhibiting blocking or blinding screen openings.

Another embodiment of the invention is a screen detachably fastened to a plate (measuring plate). which can be detachably fastened in formwork openings. The advantages of this are high rigidity of plate and screen, which have to withstand high attrition and loading during concrete placement. The plate may be detached from screen and form opening and reused elsewhere.

Another embodiment of apparatus is the measuring plate equipped with detachable closing discs or doors over openings through which the concrete inside can be sampled or tested. The advantage of this feature is that one may have access to the formwork interior without removal of the entire measuring plate and conduct tasks such as inserting pull-out shafts to pistons or carrying out impact-rebound tests.

Another embodiment connects the screen directly to the counterforce ring. This offers the advantage of sim ple, rigid design and minimum test specimen size and screen loading.

Another embodiment of invented apparatus provides for inclusion, in the counterforce ring opening, of an easily collapsable filler or resiliant disk, which will accomodate movement of the base of the frustum ruptured during the tensile-shear test. The advantage of this feature is to facilitate this test when the ring is not outside the concrete surface.

A further embodiment is a protecting disk covering the far side of ring and resilient filler. The advantage of this step is to insure that the concrete, during placement, does not prematurely indent or depress the rings resilient disk, the full volume of which should be ready to receive displacement of the moving frustum.

A further embodiment of apparatus is a guide tube, extending from the future test mass surface almost to the piston, said guide tube housing the test force transmission shaft. The advantage of this device is that the guide tube, preferably made of synthetic material, inhibits the shaft-test mass bond and facilitates lowvariation test results of tensile-shear strength.

A further embodiment of apparatus is rigid connection between protecting disk and guide tube. The advantage of this is added strength of assembly.

Another embodiment of invented apparatus is an oblong hole in piston, a hole of such dimensions that an especially shaped pull-out rod can be inserted and coupled to the piston. The advantage of this is that the rod, which can be relatively expensive for greater depths of embedment, can be withdrawn after strength determination and reused.

Another embodiment of apparatuss is a force shaft extending from the piston to near the test mass surface,

where it is fitted with a coupling piece. The advantage lies in the fact that rods which cannot be withdrawn need not extend beyond the mass surface and require cutting time and expense.

In The Drawings:

FIG. 1 is an elevation end view of a screen assembly embodiment of the invention as mounted on framework to keep course aggregate out of a designated test area;

FIG. 2 is a perspective schematic view of a screen assembly;

FIGS. 3 to 8 are elevation views, partly in section of a screen assembly in place in a concrete body showing various test fixture configurations;

FIG. 9 shows a loading disc with interlocking pull rod structure in end and side view;

FIG. 10 is an elevation view, partly in section of a test fixture arrangement in a concrete body together with a partial end view;

FIG. 11 is an elevation view, partly in section of a further test fixture arrangement in a concrete body;

FIG. 12 is a side view, partly in section and end view of a loading disk with interlocking rod structure;

FIG. 13 is an elevation view, partly in section of a further screen assembly embodiment of the invention and adjacent formwork;

FIG. 14 is an elevation view, partly in section of a force rod decoupling embodiment; and

FIGS. 15a and 15b are respectively an elevation view segment, partly in section, and top view segment of the relationships of the screen assembly, test disks and formwork of an embodiment of the invention.

In the following the invention is described in greater detail with the help of the drawings which show various embodiments of the invention. In the figures and as shown in FIG. 1, l designates the inner and 2 the outer screen rods, 3 the screen mesh aperture, 4 the formwork to which the screen is screwed with screws 5 and cross ties 6, and 7 the placing direction.

FIG. 2 shows a schematic view of the screen where for the sake of better comprehensibility the back lateral grid is omitted. The outer grid rods are drawn in full lines and the inner ones in dashed lines. During the placing the coarser grain is deviated by the outer screen rods, which are arranged parallel or in an acute angle in relation to the placing direction so that the inflow of the finer grain into the measuring space is not hindered by the coarser component. If the rods of the screen, which are perpendicular to this direction, were on the outside, they would adversely affect the flow of the coarser grain along the rods so that the inflow of the finer grain would be hampered.

FIG. 3 shows a screen in which the counterholder ring (11) borders the measuring space, and where the loading disk (10) is arranged on the surface (17) of the placed mass (14), and is loaded by a hydraulic pressure cylinder (20) with its piston (22) in the direction of the counterholder ring (11).

The load applied to the loading disk (10) is continually increased until the bond between the frustum shell (16) [defined by the loading disk circumference and the opening of the counter-holding ring (11)]and the surrounding screened mass (50) is broken so that the basis of the frustum sinks into the resilient disk (displacement disk) (12). The resiliency of the latter must not be so high that it is essentially compressed as early as the fine grain enters the measuring space. Because in this case the frustum basis would not be able to sink in during the measurement. As a consequence there would not result a load relief of the frustum and a pressure relief in the pressure cylinder, which however is required to show on the pressure gauge (18) that the bond on the frustum shell (16) is broken. As a consequence, the hydraulic cylinder would be operated later without the possibility of determining the material strength. Therefore, the resiliency of the replacement disk (12) must have an upper limit defined by the maximum pressure of the fresh concrete inside the measuring space and a lower limit defined by the lowest concrete strength to be measured.

The quotient of the measuring value indicated by the drag pointer of the pressure gauge (18) and the frustum shell (16) area results in the so-called tensional shear strength which follows approximately the direct relation to the bending strength and the compression strength: 1 l 4, as extensive experimentation has shown. The area of the frustum shell is defined by the diameter of the loading disk (10) and the diameter of the opening of the counterholder ring (11) and by the distance (23) between disk (10) and ring (11). If these dimensions are kept constant in all measuring points then the pressure gauge can be calibrated in concrete strength values so that its drag pointer directly indicates the respective momentary concrete strength (B) in kp/cm when it comes to a standstill in the moment in which the bond on the frustum shell breaks. Such a concrete strength scale is based on the below approximate formula in which (P) designates the effort (in kp) on the loading disk required for breaking the bond on the frustum shell and (M) the frustum shell area (in cm The value of the load P is the product of the value of pressure (in kp/cm at the pressure gauge (18) at the standstill of the drag pointer and the effective crosssection area of the piston inside the hydraulic pressure cylinder (20). An additional scale on the pressure guage (18) for reading the pressure (P) (in kp) exerted by the hydraulic cylinder on the loading disk (10) is useful for measurements in which for some reason the value of the frustum shell area or of the distance (23) between loading disk and counterholder ring can not be kept constant.

The FIG. 3 does not show the fixing of the screen. In measuring points without formwork the screen (1,2) can be fixed on the reinforcing shell in such a way that its upper side flush with the future concrete surface (17). In the case of a framework the upper transverse screen rods (1) are screwed to the formwork or the ends of the outer rods (2) are bent parallel to the formwork and screwed to it.

The FIG. 4 shows a version similar to that of FIG. 3 but wherein the loading disk (10) flush with the concrete surface (17). In the case of the presence of a formwork the disk (10) is screwed to it and after the placing the screw is removed. In the case of the absence of a formwork this disk is slightly clamped to a support, e.g., between two rods, 23 welded to the screen.

The FIG. 5 shows an embodiment of the invention for measuring the strength in greater depths of the placing space (14) with the same frustum dimensions. The loading disk (10) is fixed to a pressure rod (24) protected against sticking to the setting concrete by a guide tube (25) of steel or of synthetics, maintained in position by the bent ends (26) of the outer screen rods (2). Greater depths of the screen in the placing space require additional stiffening means, preferably arranged inside the screen so that they do not interfere with the deviationof the coarser grain from the screen.

The FIG. 6 shows another embodiment of the invention for measuring the strength in the depth, and where the measuring space is much smaller as compared with that of FIG. 5. Owing to the smaller size of the screen, particularly transverse to the placing direction, the screen unit is less loaded during the placing. The screen rods (1,2) are welded to the counterholder ring (11) as well to the flange (27) of the guide tube (28), which in cludes a further flange (29) for fixing the unit. When it has to be arranged in greater depths it must be anchored additionally on anchor rods. The figure shows a version according to which the loading disk is slightly clamped between two rods (123) welded to the screen and where the pressure rod (30) is not inserted until immediately before the measurement and can be unrestrictedly reused. This rod must be dimensioned against buckling as well, considering the maximum effort on the loading disk to be transferred.

The FIG. 7 shows a version in which an increasing effort by a hydraulic pull cylinder is transmitted to the loading disk (10) over a pull rod (37). The pull cylinder would prop against the counterholder ring (11). The loading disk (10) has a central bore into which the pull rod (37) is inserted which has a thickening (32) on this end.

If for the sake of an easier coupling of the pull rod (60) with the hydraulic pull cylinder the former is provided (according to FIG. 12) with such a thickening on its other end too, and the loading disk receives a further larger eccentric bore (1). The rod shaft (60) is inserted laterally into the smaller central bore (62) which opens towards the eccentric larger bores (61). The rod shaft is secured against lateral escaping from the small bore into thelarger one by a glue paste.

In an analogous way to the FIG. 4 the counterholder ring 11 can be arranged under the future concrete surface or direct on the formwork (63) according to FIG. 12. In this case, a resilient disk (13) according to the FIG. 8 is required in order to allow the sinking of the frustum basis into the opening of the counterholder ring (11).

The version shown in FIG. 8 serves for measuring the strength in practically unlimited depths of the placing space. The screen (1,2) is welded to the counterholder ring (11) the opening of which is protected against the inflow of general unscreened mass by a protecting disk (13) welded to the guide tube (33) for the pull rod (34). The guide tube (33) does not completely reach as far as the loading disk (10), where a sleeve (35) of resilient material is slipped over the tube end preventing the placed material to enter the guide tube but on the other hand allowing a small movement of the loading disk (10) when on account of a pull out force on its bond of the material along the frustum shell breaks. The loading disk (10) is slightly clamped between two rods (123) welded to the screen (1,2). The end of the pull rod (34) inside the measuring space has a crosspiece (36) which is inserted into the oblong hole (37) of the loading disk (10) according to FIG. 9, and the pull rod is interlocked with the loading disk by its being turned by app. 90. Therefore, the pull rod (34) can be reused for other measurements after its being unlocked and after its withdrawal out of the guide tube (33). On its outside end, the pull rod has a coupling or a thickening to which the piston of a hydraulic pull cylinder can be coupled, which props against the flange (38) of the guide tube (33).

FIG. 10 shows an embodiment ofthe invention where the loading disk (10) and the counterholder ring half (11) are slightly inclined towards the normal to the concrete surface (17) in such a direction that this counteracts the outside evasion tendency of the frustum half when the pull rod (40) is pulled towards the counterholder. For the same purpose, the guide rollers (41) hold the pull rod (40) against the concrete surface (17).

The FIG. 11 shows an embodiment of the invention similar to the FIG. 10. But here an additional plate (42) is arranged in the future placing space and the entrance of mass into the space between the loading disk (10) and this plate is prevented by a dummy element of the same shape as a hydraulic pressure cylinder (43) which, for the measurement, replaces the dummy element and the piston (44) of which acts on the loading disk (10). Here too the loading disk and the counterholder ring half are slightly inclined as regards the normal to the concrete surface.

The FIG. 14 shows a coupling for the force transmission rod (69) which reaches from the loading disk as far as near the mass surface (17) only, and is provided with an inner screw hole (66). A cap of plastic material (65) prevents the concrete and dust to enter this screw hole, and is put on the end of the guide tube so that it also prevents material to enter the space between it and the rod (69). For the coupling of the piston rod (68) of a hydraulic cylinder this rod has a screw bolt (67) with which the cap (65) can be perforated so that the screw bolt can be screwed into the screw hole (66). If the rod (69) is not provided for being withdrawn after measurement this solution avoids expenses for cutting protruding rod parts.

The FIG. 15 shows an example of a measuring plate (72) for arranging a number of pull rods (71) with loading disks (10) in openings (80) of the plate, which is mounted in an opening of the formwork (63). The measuring plate is provided with mounting straps (73) which exert a pressure on a pressure frame (74) and an underlying rubber sealing (83), when the measuring plate is screwed to the formwork with the aid of the screw bolts (75), which are welded to the formwork. Therewith a tight connection between formwork and measuring plate is secured. A screen (1,2) is mounted to the measuring plate (72) with the aid of the screw bolts (76) and can be unscrewed from the measuring plate for removing the latter after the concrete has hardened. On its outside, the measuring plate (72) wears a fixing metal strap (77), to which the pull rods (71 for the loading disks 10) are clamped with the aid of fixing screws (78) and fixing rings (79). The latter are welded to the fixing strap (77). The measuring plate (72) has openings (80), which are sealed by rubber disks (81 which are pressed down by metal disks (82). For measurement, the fixing screws (78) are loosened and a hydraulic pull cylinder propping against the measuring plate exerts an effort over the pull rod (71) on the loading disk (10) until the frustum shell (84) loses its bond with the surrounding mass, so that the frustum basis sinks a little into the rubber plate (81).

In the present case, the counterholder is materialized by the border of the opening (80) of the measuring plate. The depth of the screen is adjusted by aspacing tube (85). The measuring plate can be provided with other openings for strength measurements with the rebound hammer and with closing disks for its openings.

The embodiments shown in the figures demonstrate the application of the method of filtering out the coarser grain to the method of breaking the bond between the shell of the concrete frustum and the surrounding mass. But this filtering method can also be favourably applied to other strength testing methods, e.g., to the various kinds of test hammers (rebound and impression method.) and to the drill core method, i.e., to methods which can not be used until higher strength values have been reached. The retaining of the coarser grain component from the measuring space especially reduces the dispersion of the measuring values of these methods, and the results are always on the safe side, as extensive investigations have shown.

What we claim is:

l. The method of testing the strength of a body comprising a setting substance, such as concrete, having a separable component such as a coarse grain aggregate affecting the accuracy of said testing, comprising the steps of, selecting in said body a test site portion, eliminating said separable component from the test site, and testing the strength of said body located within said test site.

2. The method defined in claim 1 wherein the separable component is eliminated at said test site by screening out coarser components at a time prior to forming said body, and said body is formed thereafter with the coarser components therein about and in contact with said test site portion.

3. The method defined in claim 1 wherein the step of eliminating said component comprises the step of placing screens for shielding the test site to pass only finer grain components thereinto.

4. The method definedin claim 3 including the step of mounting formwork to confine materials of said body, and removably mounting said screens on said formwork for detaching prior to removal of the formwork from said body.

5. The method defined in claim 4 wherein the formwork defines openings at the test site, and including the step of mounting measuring plates at the openings as part of said framework for positioning said removably mounted screens thereon.

6. The method defined in claim 1 wherein said testing comprises the steps of mounting at the test site a disk and a counterholder with an opening larger than the disk therein spaced substantially parallel to each other, and exerting an increasing force between the disk and counterholder to break the bond between the test site and surrounding body to form thereby a truncated cone.

7. The method defined in claim 6 including the step of arranging the disk and counterholder to define an aperture angle of said cone'between 55 and 90.

8. The method defined in claim 7 including the step of limiting said angle to the range of 63 and 73.

'9. The method defined in claim 6 wherein the body presents a surface area, including the step of placing the counterholder nearer the surface area than the disk.

10. The method defined in claim 6 wherein the body presents a surface area, including the step of placing the disk nearer the surface area than .the counterholder. 1 A

11. The method defined in claim 6 wherein the body presents a surface area, including the step of arranging the disk and the counterholder so that a line normal their parallel surfaces is normal to said surface area.

12. The method defined in claim 6 wherein the body presents a surface area, including the step of arranging the disk and the counterholder so that a line normal their parallel surfaces is inclined to said surface area.

13. The method defined in claim 6 wherein the body presents a surface area and the step of exerting the force comprises guiding a pull rod substantially parallel to said surface area.

14. The method defined in claim 6 wherein the body presents a surface area, comprising the step of forming a recess in said surface area, and wherein said force is exerted by means placed in said recess.

15. The method defined in claim 6 including the steps of placing formwork to define said body, attaching said disk and counterholder to said formwork prior to forming said body, and removing said disk and counterholder from said formwork prior to removing said formwork from said body.

16. The method defined in claim 15 wherein the formwork defines an opening, and including the step of attaching a mounting plate to the formwork at said opening, wherein said step of attaching said disk and counterholder to said framework comprises attaching them to said mounting plate.

17. A testing device for testing the strength of a body comprising a setting substance, such as concrete, having a separable component, such as a coarse grain aggregate affecting the accuracy of said testing, comprising in combination, a screen arranged for defining a test site portion within the body to separate said separable component from the test site within the screen, a resilient disk and a substantially parallel counterholder defining an aperture larger than said disk arranged at said test site portion with said screen connected to said counterholder, and said disk disposed in said aperture.

18. A testing device as defined in claim 17 including formwork for defining the body, said formwork defining an opening, a plate mounted at said opening, and said screen being fixed to said plate.

19. A testing device as defined in claim 18 wherein said plate defines openings therein, detachable closing plates close said openings, and including force exerting means passed through said openings.

20. A testing device as defined in claim 17 wherein the resilient disk has an upper limit of resiliency defined by a negligible compression of the disk to the pressure of said body thereon during setting.

21. A testing device as defined in claim 17 wherein the lower limit of resiliency of said resilient disk is defined by the lowest desired measuring value of the substance strength.

22. A testing device as defined in claim 17 including means protecting said aperture of said counterholder to keep any body portion with said separable component therein from passing into said test site portion.

23. A testing device as defined in claim 17 including a test disk placed in the substance at said test site portion at a position displaced from said resilient disk with a guide tube extending from the test disk for receiving a force transmission rod to displace it toward said resilient disk.

24. A testing device as defined in claim 17 including means protecting said aperture of the counterholder to keep any body portion with said separable component therein from passing into said test site.

25. A testing device as defined in claim 17 including a test disk placed in the substance at said test site portion spaced from said resilient disk defining therein an oblong opening dimensioned for passing a portion of a force exerting rod.

26. A testing device as defined in claim 17 including a test disk placed in the substance at said test site portion spaced from said resilient disk and having a force transmission rod thereon reaching toward the surface of said body.

27. A testing device as defined in claim 17 including a counterholder ring and a test disk attached to a piston shaft, said test disk being embedded in said test site portion spaced from said resilient disc and said ring being of greater size than said test disk to thereby cause a conic frustum rupture of said substance toward said resilient disk when the forces exceed the strength of said substance.

28. A testing device as defined in claim 27 including a guide tube detachably fastened to said test disk for guiding said shaft.

29. The method of testing the momentary strength of a body comprising a setting substance, such as concrete, comprising the steps of mounting formwork to confine materials of said body, removably mounting on said formwork an assembly which includes a measuring disk to be embedded in said substance, pouring said materials into said formwork to form said body and envelop said disk, detaching said assembly from said formwork, removing said formwork from said body to leave exposed a portion of said assembly accessible to pull said disk from said body to test the strength of said body, and applying a measurable load by means of said assembly to said disk to break it away from said body thereby identifying at the break away point the strength of said body. 

1. The method of testing the strength of a body comprising a setting substance, such as concrete, having a separable component such as a coarse grain aggregate affecting the accuracy of said testing, comprising the steps of, selecting in said body a test site portion, eliminating said separable component from the test site, and testing the strength of said body located within said test site.
 2. The method defined in claim 1 wherein the separable component is eliminated at said test site by screening out coarser components at a time prior to forming said body, and said body is formed thereafter with the coarser components therein about and in contact with said test site portion.
 3. The method defined in claim 1 wherein the step of eliminating said component comPrises the step of placing screens for shielding the test site to pass only finer grain components thereinto.
 4. The method defined in claim 3 including the step of mounting formwork to confine materials of said body, and removably mounting said screens on said formwork for detaching prior to removal of the formwork from said body.
 5. The method defined in claim 4 wherein the formwork defines openings at the test site, and including the step of mounting measuring plates at the openings as part of said framework for positioning said removably mounted screens thereon.
 6. The method defined in claim 1 wherein said testing comprises the steps of mounting at the test site a disk and a counterholder with an opening larger than the disk therein spaced substantially parallel to each other, and exerting an increasing force between the disk and counterholder to break the bond between the test site and surrounding body to form thereby a truncated cone.
 7. The method defined in claim 6 including the step of arranging the disk and counterholder to define an aperture angle of said cone between 55* and 90*.
 8. The method defined in claim 7 including the step of limiting said angle to the range of 63* and 73*.
 9. The method defined in claim 6 wherein the body presents a surface area, including the step of placing the counterholder nearer the surface area than the disk.
 10. The method defined in claim 6 wherein the body presents a surface area, including the step of placing the disk nearer the surface area than the counterholder.
 11. The method defined in claim 6 wherein the body presents a surface area, including the step of arranging the disk and the counterholder so that a line normal their parallel surfaces is normal to said surface area.
 12. The method defined in claim 6 wherein the body presents a surface area, including the step of arranging the disk and the counterholder so that a line normal their parallel surfaces is inclined to said surface area.
 13. The method defined in claim 6 wherein the body presents a surface area and the step of exerting the force comprises guiding a pull rod substantially parallel to said surface area.
 14. The method defined in claim 6 wherein the body presents a surface area, comprising the step of forming a recess in said surface area, and wherein said force is exerted by means placed in said recess.
 15. The method defined in claim 6 including the steps of placing formwork to define said body, attaching said disk and counterholder to said formwork prior to forming said body, and removing said disk and counterholder from said formwork prior to removing said formwork from said body.
 16. The method defined in claim 15 wherein the formwork defines an opening, and including the step of attaching a mounting plate to the formwork at said opening, wherein said step of attaching said disk and counterholder to said framework comprises attaching them to said mounting plate.
 17. A testing device for testing the strength of a body comprising a setting substance, such as concrete, having a separable component, such as a coarse grain aggregate affecting the accuracy of said testing, comprising in combination, a screen arranged for defining a test site portion within the body to separate said separable component from the test site within the screen, a resilient disk and a substantially parallel counterholder defining an aperture larger than said disk arranged at said test site portion with said screen connected to said counterholder, and said disk disposed in said aperture.
 18. A testing device as defined in claim 17 including formwork for defining the body, said formwork defining an opening, a plate mounted at said opening, and said screen being fixed to said plate.
 19. A testing device as defined in claim 18 wherein said plate defines openings therein, detachable closing plates close said openings, and including force exerting means passed through said openings.
 20. A testing device aS defined in claim 17 wherein the resilient disk has an upper limit of resiliency defined by a negligible compression of the disk to the pressure of said body thereon during setting.
 21. A testing device as defined in claim 17 wherein the lower limit of resiliency of said resilient disk is defined by the lowest desired measuring value of the substance strength.
 22. A testing device as defined in claim 17 including means protecting said aperture of said counterholder to keep any body portion with said separable component therein from passing into said test site portion.
 23. A testing device as defined in claim 17 including a test disk placed in the substance at said test site portion at a position displaced from said resilient disk with a guide tube extending from the test disk for receiving a force transmission rod to displace it toward said resilient disk.
 24. A testing device as defined in claim 17 including means protecting said aperture of the counterholder to keep any body portion with said separable component therein from passing into said test site.
 25. A testing device as defined in claim 17 including a test disk placed in the substance at said test site portion spaced from said resilient disk defining therein an oblong opening dimensioned for passing a portion of a force exerting rod.
 26. A testing device as defined in claim 17 including a test disk placed in the substance at said test site portion spaced from said resilient disk and having a force transmission rod thereon reaching toward the surface of said body.
 27. A testing device as defined in claim 17 including a counterholder ring and a test disk attached to a piston shaft, said test disk being embedded in said test site portion spaced from said resilient disc and said ring being of greater size than said test disk to thereby cause a conic frustum rupture of said substance toward said resilient disk when the forces exceed the strength of said substance.
 28. A testing device as defined in claim 27 including a guide tube detachably fastened to said test disk for guiding said shaft.
 29. The method of testing the momentary strength of a body comprising a setting substance, such as concrete, comprising the steps of mounting formwork to confine materials of said body, removably mounting on said formwork an assembly which includes a measuring disk to be embedded in said substance, pouring said materials into said formwork to form said body and envelop said disk, detaching said assembly from said formwork, removing said formwork from said body to leave exposed a portion of said assembly accessible to pull said disk from said body to test the strength of said body, and applying a measurable load by means of said assembly to said disk to break it away from said body thereby identifying at the break away point the strength of said body. 